No, it's criminal, to be punishable by death, and a battlefield execution if caught so in combat. Torture has no place in combat. In order to torture somebody, one already displays they have control of a situation. If you have not changed the enemy's willpower, then you imprison him as a POW. If he disobeys and commits criminal behavior, then an appropriate punishment is levied accordingly. This doesn't mean people don't get killed in combat. It means that those who confuse combat for a realm without reason are doomed to die by their chaos.
A skilled sniper has unbelievable, awesome control over the "situation." Yet exploiting that control is no crime.
Unfortunately, terrorists don't meet the threshold of qualifying as beligerents under the Geneva Convention. Accordingly, they cannot be POWs.